Method and apparatus for microwave assisted chemical reactions

ABSTRACT

A method of microwave assisted chemical reaction includes providing a microwavable reaction vessel which contains at least one material in a sample. The sample is heated by microwave energy to elevate the temperature of the reagent and at least partially volatilize the sample to establish a gas phase within the vessel followed by positive cooling of the gas phase to reduce the temperature and responsively reduce the pressure of the gas phase without effecting substantial cooling of the liquid phase. The method may involve employing cooling exteriorly of and adjacent to the gas phase containing portion of the vessel or cooling by means of a coolant flowing within coils disposed in the interior of the vessel or both. The process is preferably a continuous process. The apparatus may be a vessel transparent to microwave energy for receiving the sample. The vessel has space overlying the liquid phase containing portion for a gas phase. Structures for cooling means for positively cooling the gas phase to reduce the pressure of the gas phase without effecting substantial cooling of the reagent are provided. These structures for cooling may be contained within the vessel, exteriorly of the vessel or modification of the vessel configuration to facilitate gas phase cooling or combinations thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/357,097, filed Dec. 15, 1994, entitled "Method and Apparatusfor Microwave Assisted Chemical Reactions," now abandoned, which was acontinuation of U.S. patent application Ser. No. 08/127,263, filed Sep.24, 1993, entitled "Method and Apparatus for Microwave Assisted ChemicalReactions," now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of microwave assisted chemicalreactions, such as sample preparation, synthesis, derivatization orextraction which involves reduced pressure within the vessel andassociated apparatus for accomplishing this objective.

2. Description of the Prior Art

The use of microwave energy in analytical chemistry to provide heat toassist chemical reactions has been known for approximately 15 years.See, generally, Mingos et al., Applications of Microwave DielectricHeating Effects to Synthetic Problems in Chemistry, Chem. Soc. Rev.1991, 20, pp. 1-47.

It has been known to employ such microwave heating in samplepreparation. See, Kingston et al., Comparison of Microwave VersusConventional Dissolution of Environmental Applications, Spectroscopy 7(9) November/December 1992, pp. 20-27. One approach involves anopen-vessel approach in which the result is achieved with the assist ofmicrowave heating. An alternate approach is the so called"closed-vessel" microwave sample preparation.

It has been know to use microwave energy for various types ofenvironmental processes. For example, microwave energy, such as thatproduced by a nominal or high intensity microwave oven, has beenemployed to extract pesticides from sediment samples. See, Onuska etal., Extraction of Pesticides from Sediments Using a MicrowaveTechnique, Chromatographia, Vol. 36, pp. 191-194 (1993). Microwaveheating has also been employed in effecting hydrolysis of proteins. See,Margolis et al., The Hydrolysis of Proteins by Microwave Energy, Journalof Automatic Chemistry, Vol. 13, No. 3, pp. 93-95 (May/June 1991).

It has also been known to employ microwave energy in a closed vesseldigestion system wherein a closed Teflon PFA vessel has an organicsample, an inorganic sample or a combination subjected to aciddecomposition under the influence of microwave energy. See, Kingston etal., Microwave Energy for Acid Decomposition at Elevated Temperaturesand Pressures Using Biological and Botanical Samples, Anal. Chem., 58,pp. 2534-2541, (October, 1986).

In such closed vessel microwave sample preparation techniques,typically, one or more materials which will become the sample are mixedor dissolved in a suitable liquid reagent. The liquid reagent occupies aportion of the volume of the relatively small vessel and is subjected tochemical alteration under the influence of the microwave heating,thereby creating a gas phase in addition to the liquid phase within thevessel. The microwave heating results in increased temperatures andpressures within the vessel which can present a potential safety hazardthrough vessel failure. The increased temperature is required foradvancement of the reaction rate, but the pressure is a property of theheat flow characteristics of the vessel and microwave interaction.

It has been known to control heat loss from the vessel by providing ajacket of thermal insulation around the vessel which also acts tostrengthen the vessel. See, generally, Mingos et al., Applications ofMicrowave Dielectric Heating Effects to Synthetic Problems in Chemistry,Chem. Soc. Rev., 1991, 20, pp. 1-47 and Chapter 6, Introduction toMicrowave Sample Preparation Theory and Practice by Kingston et al.,American Chemical Society, 1988, pp. 93-154.

U.S. Pat. No. 5,215,715 discloses a method of digesting materials whichare dispersed in an acid digesting medium, which dispersion is subjectedto microwave heating in a first chamber and then both the gas and liquidphases of the dispersion are cooled in another chamber. There is nosegregated cooling of the gas phase while heating the liquid phase.There is also no recognition of the pressure relationship between thegas phase and liquid phase during microwave radiation.

In prior art practices, pressure within the vessel has been permitted toform at whatever natural level occurred due to the specific reagents,temperature, reaction products, microwave interaction and heat flow ofthe vessel.

There remains, therefore, a very real and substantial need for a moreefficient and safe means of microwave sample preparation in a closedvessel.

SUMMARY OF THE INVENTION

The present invention has solved the above-described problems byproviding a method and apparatus wherein a microwavable reaction vesselis provided with a liquid reagent mixture and/or sample. For convenienceof reference herein, both of these categories and any similar materialsto be processed will be referred to as a "sample." The sample is heatedso as to elevate the temperature thereof to establish at least partialvolatization of the sample and thereby create a gas phase overlying theliquid reagent within the vessel. The gas phase is positively cooled toreduce the temperature in the gas phase and, responsive to saidtemperature reduction, reducing the pressure without effectingsubstantial cooling of the liquid reagent.

The cooling of the gas phase may be effected by providing channels forcoolant flow exteriorly of the vessel or coolant flow within the vesselwithin coils or both. In this manner, the temperature and pressure ofthe gas phase are reduced in the preferred practice of the invention,while the coolant flowing in the cooling conduits, whether they aredisposed interiorly or exteriorly of the vessel or both, does notdirectly cool the liquid reagent.

The apparatus for practicing the method preferably includes a vessel,such as a vessel or vessel liner made from a suitable polymer orfluoropolymer, such as polytetrafluoroethylene, TFM or perfluoroalkoxy,which is transparent to microwave energy and receives the liquid reagentmixture and/or sample. The vessel may also utilize an outer casing of adifferent material, such as polyetherimide, glass filled polyetherimide,and other suitable materials. The vessel has additional capacity for thegas phase. Cooling means provide for positive cooling of the gas phaseto reduce the temperature and pressure of the gas phase. The coolingmeans has passageways for the flow of coolant. The passageways may bedisposed exteriorly of the vessel and adjacent to the outer walls of thevessel with the passageways not being disposed adjacent to the sample orliquid reagent containing portion of the vessel. In another embodiment,the passageways are coils disposed within the gas phase portion of thevessel.

It is an object of the present invention to provide a method andapparatus for closed vessel microwave assisted chemical reactions whicheffectively reduces the pressure in the gas phase within the vessel.

It is another object of the invention to provide such a system whereinthe pressure reduction in the gas phase is effected through positivecooling to reduce the temperature thereof.

It is another object of the present invention to provide such a systemwhich may be employed in microwave digestion and reaction bombs.

It is a further object of the present invention to provide such a systemwhich is employed in preparing chemical samples for later analysis.

It is a further object of the invention which permits microwave heatingof the sample to elevate its temperature simultaneous with positivecooling of the gas phase.

It is yet another object of the invention to provide such a system whichis adapted to accomplish sample preparation in a much more rapid mannerthan those previously known.

It is a further object of the present invention to provide such a systemwhich will contribute to increased durability of the vessels.

These and other objects of the invention will be more fully understoodfrom the following description of the invention on reference to theillustrations appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional illustration of a form of vesselof the present invention having external cooling passageways.

FIG. 2 is a schematic cross-sectional illustration of a form of vesselof the present invention having internal cooling passageways.

FIG. 3 is a schematic flow chart showing a continuous system of thepresent invention.

FIG. 4 is a comparison of reaction conditions in Teflon and insulatedvessels.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Microwave vessels employed in chemical reactions, such as samplepreparation, synthesis, derivatization and extraction generally are ofrelatively moderate size and may have an interior volume of about 1 mLto 500 mL and preferably in the range of about 1 mL to 125 mL. Thevessels may have any desired configuration, but are frequently generallycylindrical in shape. They may be made of Teflon (tetrafluoroethylene,PFA or TFM or PTFE) or other fluorinated carbon plastics with aremovable lid adapted to seal in place as by threaded or pressure fittedsecurement to maintain the desired amount of pressure, which for thistype vessel, might be in the order of up to about 10 atmospheres.Another type of vessel would have a plastic casing for rigidness andpressure stability with a Teflon, plastic or quartz liner for chemicalinertness and be adapted to withstand pressures of about 5 to 20atmospheres. In this latter category, the vessel may be designed so asto withstand pressures of 40 to 100 atmospheres.

Closed vessel digestion will generally achieve higher temperaturesbecause the boiling point of the reagent is raised by the pressureproduced within the vessel. The higher temperature in the closed vesselwill, however, greatly reduce the time required for reaction. The closedvessel also resists evaporation and there is, therefore, no need to addreagent to maintain the desired volume.

The vessels are effectively transparent to microwave energy so as topermit them to be introduced into a microwave oven and the reagents andsamples contained in them to be heated to the desired temperature. Asthe liquid reagent containing one or more materials is heated, a gasphase is formed through the vaporization of the solvent and/or thechemical materials. The sample or samples will generally be mixed with aliquid reagent which may, for example, be nitric acid employed inmicrowave-heated digestions. In order to maintain pressure levels withinthe desired ranges of safety and contribute to durability of thevessels, as well as achieving the desired temperature which is mostbeneficial for the chemical reaction contemplated, the present inventionprovides positive cooling to the gas phase contained within the vesselwhile resisting effecting meaningful cooling of the liquid reagent.

For a given liquid reagent, the absorption of microwave energy can becalculated at a specific frequency employing Equation 1. ##EQU1##wherein: P=is the apparent power absorbed by the sample in watts (W),(W=joules/sec);

K=is the conversion factor for thermochemical calories/sec to W, whichis 4.184;

C_(p) =is the heat capacity, thermal capacity, or specific heat(cal./g.ΔC);

m=is the mass of the sample in grams (g);

ΔT=is T_(f), the final temperature minus T_(i), the initial temperature(ΔC); and

t=is the time in seconds (s). ##EQU2##

In the event that no energy is permitted to escape from the vessel, thefinal temperature can be determined by equation 2.

As shown in Equation 3, a lower temperature is achieved if energy ispermitted to escape. This escape can be primarily from the gas phase asit has the greatest area of cool vessel wall to contact.

In the present invention, active cooling of the gas phase serves toreduce the gas phase pressure. If desired, the microwave energy appliedto the liquid phase sample may be increased to compensate for thethermal energy losses to the gas phase.

Referring now more specifically to FIG. 1 wherein there is shown aclosed microwave reaction vessel which may be adapted for use withautomation or a robot as distinguished from individual human handling,if desired. There is shown a vessel consisting of a liner 2 which may becomposed of a suitable fluorinated carbon plastic, such astetrafluoroethylene which is sold under the trade designation "Teflon"or other material having suitable strength, microwave transparency, andchemical inertness. The vessel liner 2 has a threaded closure 4intimately secured in sealing relationship to the liner 2. The closure4, in the form shown, has a pair of upwardly projecting, threadedlysecured port defining members 5, 6 to which apertured closures 7, 8,respectively, are secured. While these port closures 7, 8 may be closedoff if desired, in the illustrated embodiment temperature probes 10, 12,respectively, extend into the vessel 2 to different depths. These probes10, 12 may be of any conventional type and are sealingly secured to theport closures 7, 8.

Positioned in surrounding relationship with respect to liner 2 is anouter wall or casement 20 which is in intimate surface-to-surfacecontact with the exterior of the vessel 2 and closure 4. The casement 20may be provided in multiple pieces (not shown) assembled around thevessel by any desired means known to those skilled in the art. Thevessel 2, closure 4, and outer wall 20 are preferably of generallycylindrical configuration. The outer wall or casing 20 has an inwardlyopen continuous helical groove 22 which cooperates with exterior of thevessel liner 2 and closure 4 to create a continuous coolant flowpassageway. The passageway is spaced (measured along the vessellongitudinal axis) from the sample liquid reagent received portion 30 ofthe vessel. A coolant entry channel 24 is defined within casement 20 andis in communication with passageway 22. Coolant is discharged throughexit channel 26. The coolant will preferably be captured as it emergesfrom channel 26 and subjected to a heat exchanging temperature reductionafter which it may be reintroduced into coolant entry channel foranother cycle of operation. The coolant may be microwave non-absorbing,moderately absorbing, or strongly absorbing material that may be in agas or a liquid phase.

If desired, the coolant passageways may be provided in other ways. Forexample, such as by a single ring, which is inwardly open to provide anannular passageway in cooperation with or adjacent to the exterior ofthe vessel. Also, an axially elongated single ring or a plurality ofsuch rings either interconnected or individually supplied with coolantmay be employed.

Referring now to FIG. 2 in greater detail there is shown a microwavablevessel 40 having threadedly and sealingly secured thereto a closure 42which has a pair of externally threaded ports 44, 46 to which aresecured threaded sealing closures 48, 50 respectively. The liquidreagent mixture or sample 54 is contained within the lower portion ofthe vessel interior and the gas phase 56 appears thereabove. A coolantcoil 60 is received within the interior vessel 40 and has an entry end62 and a discharge end 64. In effecting cooling of the gas phase 56without effecting substantial cooling of the liquid reagent mixture 54,coolant is permitted to flow into entry 62, assume a heat exchanginginteraction with the gas phase and then emerge at an elevatedtemperature at discharge end 64. The coolant coming out of end 64 issubsequently subjected to a heat exchanging process wherein thetemperature of the coolant is reduced after which the coolant isreintroduced through entry 62. It will be appreciated that, in thismanner, continuous cooling of the gas phase will be effected to therebyreduce the pressure within the gas phase 56. If desired, coils ofadditional length or multiple coils having separate entries may beemployed. If desired, radiator structures may be employed in the vesselinterior in lieu of the coil or coils.

It will be appreciated that the embodiment shown in FIGS. 1 and 2 arenot mutually exclusive and that the coil or coils employed in connectionwith the embodiment of FIG. 2 may be employed in addition to thepassageway containing outer wall 24 of FIG. 1 in order to achieve thedesired degree of temperature reduction of the gas phase andcorresponding reduction of pressure in the vessel interior.

The partial traditional equilibrium pressures and the partial pressuresof the reagents and sample and reaction byproducts do not hold in thissystem as equilibrium of temperature between liquid and gas phases isnever reached. Condensation of several components may occur reducing thepartial pressure of one or more thus reducing the total pressure in thevessel. A dynamic nonequilibrium condition is established that is uniqueto microwave reagent closed vessel systems such as these and is a newrelationship that is being employed to produce these new reactionconditions.

Referring now to FIG. 3, there is shown schematically a block diagram ofa continuous or semi-continuous flow system of the present invention.The gas phase portion of vessel 80 receives coolant through pipe 82 bymeans of pump 84. After the coolant absorbs heat from the gas phasecontained within vessel 80, the elevated temperature coolant emergesthrough pipe 90 and enters heat exchanger 92 wherein heat is withdrawnand the coolant is reduced to a temperature desired for introductioninto the gas portion of vessel 80. The reduced temperature coolantemerges from the heat exchanger 92 and is carried by pipe 94 to pump 84for reintroduction into vessel 80.

Referring to FIG. 4, there is shown a plot of temperature in degreescentigrade and pressure in atmospheres as related to time. It compares athermally insulated vessel with a thermally uninsulated vessel, i.e., aTeflon vessel. The difference in pressure inside the vessels is due tothe loss of thermal energy in the gas phase. For example, the pressureof 6* 10 mL of concentrated nitric acid irradiated at 574 watts for 10minutes at 180° C. is about 40 psi in the insulated vessel and is onlyabout 8 psi in the uninsulated vessel. The absorption of microwaveenergy which can be calculated from equation 1 is the same for a givenliquid.

EXAMPLE

In order to enhance the understanding of the invention, an example willbe provided. A closed microwave vessel having an interior volume of 120mL is provided with 20 mL of nitric acid mixed with a 0.5 gram livertissue (material) in a closed vessel acid digestion process. The vesselwas exposed to 500 watts of microwave energy for a period of 10 minutesto establish a liquid temperature of 190° C. and a liquid partialpressure inside the vessel of 620 psi without cooling. When a similarsituation is constructed with cooling of the gas phase, there wasestablished a pressure with the acid and digestion products of 120 psiinside the vessel. This demonstrates positive cooling by a method of thepresent invention employing a method of air coolant to produce after 10minutes a gas phase temperature of 130° C. and a gas phase partialpressure of 120 psi without effecting a substantial reduction in theliquid phase temperature. A 650 watt power was applied in the secondexample to maintain the liquid temperature at 190° C. As a result, theacid digestion was effected while reducing the vessel pressure by 500psi.

The coolant may be a gas or liquid with or without entrained solids, andis preferably transparent to microwave energy. Among the preferredcoolant, materials are one or more materials selected from the groupconsisting of air, CO₂, freon, gaseous N₂ and liquid N₂.

The system of the present invention builds upon and enhances certainscientific principles as applied to solve a particular problem. Theunique nature of microwave interaction and two distinct heat transfermechanisms permits the cooling of the gas phase while continuing to heatthe liquid phase. Heating a liquid in a microwave field is commonlyreferred to as dielectric loss. The two primary mechanisms are dipolerotation and ionic conduction. See, generally, Kingston, H. M. andJassle, L. B., Eds., "Introduction to Microwave Sample Preparation:Theory and Practice," ACS Professional Reference Book, American ChemicalSociety, Washington, D.C., 1988, pp. 9-15. Ionic conduction is theconductive migration of dissolved ions in the applied electromagneticfield. Dipole rotation is the alignment, due to the electric field, ofmolecules that have permanent or induced dipole moments. When a moleculevaporizes and is converted to the gas phase, from the liquid phase,charged ions are left in the liquid phase, thereby eliminating thisheating mechanism. In addition, rotation of the molecule in the gasphase does not efficiently transfer heat, as rotation without collision,does not add heat to the gas phase. Gas molecules frequently collidewith the surfaces of the vessel. These surfaces are not heated bymicrowave energy and are actively cooled, thereby cooling the gas phase.The vessel is generally made of a material which is usually essentiallymicrowave transparent. The gas phase is not efficiently heated by themicrowave field even though the gas phase and liquid phase both exist inthe same microwave field. These heating conditions are unique to themicrowave environment. The present invention employs the ability to coolthe gas phase while continuing to heat the liquid phase in thisenvironment. The present invention involves intentionally cooling thegas phase while heating the liquid phase to effect the reduction of theinternal vessel pressure while maintaining a relatively high liquidtemperature in which various chemical reactions are conducted.

It will be appreciated, therefore, that the present invention provides amethod and apparatus for pressure control and reduction inmicrowave-assisted chemical reaction systems. This is accomplishedthrough positive cooling of the gas phase which is in contact with theliquid phase in the chemical reaction vessels without effectingsignificant reduction in temperature of the liquid phase. The positivecooling of the gas phase facilitates corresponding pressure control ofthe gas phase in order to achieve the desired chemical or physicalparameters during and following the reaction period. The reactions inthe liquid phase can, therefore, be carried out without undesiredinterference as a result of the positive cooling of the gas phase. Thepractice of the present invention will generally reduce the pressure inthe gas phase about 50 to 95 percent and preferably about 60 to 90percent. If desired, positive cooling action may be terminated orregulated when the desired gas phase pressure has been attained.

It will be appreciated that the present invention permits efficientthermally activated chemical reactions to occur at the desiredtemperature, while facilitating a reduction in pressure within thevessel at that temperature. This facilitates improved processefficiency, safety and durability. Improvement of the durability of thevessel is achieved through maintaining the integrity by resistingoverheating of the casing in double walled vessels. Also, in theembodiment of FIG. 1, the coolant may serve to carry away sample orreaction products that might become trapped between the outer wall 20and the vessel liner 2.

Also, if desired, the vessel might be formed with partially hollowoutwardly projecting fins or ribs to facilitate radiation loss of heatfrom the gas phase. In the alternative, multi-walled vent openings maybe provided in the outer wall to enhance cooling of the gas phase.

A plurality of circumferentially spaced, axially oriented ribs may beprovided within the gas phase region of the vessel, but not in theliquid phase portion. such a construction will be deemed positivecooling within the context of the present invention.

In addition to the foregoing the turntable onto which the vessel isplaced may be cooled. The hollow turntable top might have a recess whichreceives an upper portion of the vessel in intimate contact therewith.Coolant may be circulated within the hollow turntable top.

While not the preferred practice of the invention, if desired, gas maybe withdrawn from the gas phase of the vessel, cooled and subsequentlyreturned to the gas phase of the vessel.

The vessel may be a container that holds volumes from about 50 mL to 500mL or may be an elongated tube which is closed to the atmosphere and inwhich the sample flows through the microwave field.

An elongated tube may have the sample and gas phase moving by themicrowave source and cooling means so as to permit both heating of thesample and cooling of the gas phase which would be present in the sealedtube. As this embodiment would involve commingling of the liquid sampleand gas phase, it is not the preferred embodiment.

It will be appreciated that the present invention may be employedadvantageously with a wide variety of materials and end uses. Thefollowing examples will illustrate some advantageous uses. Among thespecific end uses for which the sample preparation, method and apparatusof the present invention may be employed are microwave assisteddecomposition, synthesis, derivatization and/or extraction or leaching.The invention may be employed to perform mineral acid decompositionswhile cooling the acid vapor to reduce the temperature and responsivelythe pressure of the decomposition system. Also, organic extraction withorganic solvents may be performed while cooling the gas phase to reducethe pressure of the overall reaction.

The invention may be employed to perform organic or inorganic synthesiswith solvents while cooling the gas phase to reduce the pressure duringsynthesis.

The invention may also be employed to perform hydrolysis on a proteinwith a solvent mixture including hydrochloric acid and cooling the gasphase to effect a reduction in pressure during hydrolysis. Another useis drying to condense components of the vapor phase.

In some instances, the gas phase may be cooled to resist temperaturedamage to the material out of which the inner liner or outer casings aremade, such as polyetherimide, for example. The invention may also beemployed with azeatropes, as well as aqueous materials.

Uses in environmental, biological, medical and industrial fields will bereadily apparent to those skilled in the art.

The invention may be employed with all types of microwave systemsincluding, for example, cavity-type microwave systems, focused microwavesystems, flow and stop flow microwave systems, and antenna transmittedmicrowave cavities.

With respect to the liquid temperature, if desired, one may operate at ahigher liquid temperature with no increase in vessel internal pressureor at similar liquid temperatures with a decrease in pressure.

The invention further facilitates resisting undesired escape of thevolatile elements, molecules, and compound losses when opening vesselsto the atmosphere and condensing of these from the gas phase.

Whereas particular embodiments of the invention have been describedherein for purpose of illustration, it will be evident to those skilledin the art that numerous variations of the details may be made withoutdeparting from the invention as defined in the appended claims.

I claim:
 1. A method of microwave assisted chemical reactioncomprising,providing a closed microwavable reaction vessel having asample contained therein, heating said sample with microwave energy toelevate the temperature thereof and at least partially volatilize saidsample to establish a gas phase within said closed reaction vessel, andsimultaneously with said heating of said sample positively cooling saidgas phase within said reaction vessel to reduce the temperature andpressure of said gas phase without effecting substantial cooling of saidsample, whereby undesired vessel failure due to pressure buildup withinsaid gas phase will be resisted.
 2. The method of claim 1includingeffecting said cooling by flow of a coolant which receives heatfrom said gas phase.
 3. The method of claim 2 includingproviding saidvessel with an exterior wall which defines cooling flow passageways onportions of the exterior of said vessel, effecting flow of said coolantin said passageways to receive heat conducted from said gas phasethrough said vessel wall and into said coolant, subsequently coolingsaid coolant, and subsequently effecting flow of said coolant throughsaid passageways to receive heat from said gas phase.
 4. The method ofclaim 2 includingproviding coolant flow coils within said vessel, andeffecting flow of said coolant through said coils to withdraw heat fromsaid gas phase.
 5. The method of claim 3 includingestablishing saidcoolant flow solely in regions of said vessel exterior which are notadjacent to said liquid sample.
 6. The method of claim 4includingestablishing said coolant flow in said coil solely in portionsof the gas phase and not in said liquid sample.
 7. The method of claim 2includingemploying a vessel having an internal volume of about 1 mL to500 mL.
 8. The method of claim 7 includingemploying a vessel or vesselliner made of a fluoropolymer material.
 9. The method of claim 2includingemploying as said coolant material, a material which issubstantially transparent to microwave energy.
 10. The method of claim 1includingforming said process as a continuous or semi-continuous flowprocess wherein coolant which receives heat from said gas phase issequentially cooled and returned to said vessel to absorb additionalheat.
 11. The method of claim 1 includingemploying a coolant in heatexchange relationship with said gas phase to effect said positivecooling, and after positively cooling said gas phase reducing thetemperature of said coolant and then returning said coolant to cool saidgas phase again.
 12. The method of claim 1 includingreducing thepressure in said gas phase by about 60 to 90 percent by said process.13. The method claim 1 includingproviding outwardly projecting ribs inat least a portion of said vessel adjacent to said gas phase tofacilitate cooling of said gas phase.
 14. The method of claim 1includingemploying as said vessel an elongated sealed tube which isdisposed within a microwave field and effecting flow of said sample insaid tube.